HARTFORD — A former physics teacher and coach at Miss Porter's School in Farmington who is serving an 18-month prison sentence for sexually assaulting one of his students faces more time in prison.

Joseph Prem Rajkumar, 44, pleaded guilty Tuesday in U.S. District Court in Hartford to a single count of attempted receipt of child pornography. The charge relates directly to his misconduct with the Miss Porter's student.

According to the government, in approximately April 2011, Rajkumar pressured one of his students into creating an anonymous email account that did not contain her name. Rajkumar, who already had such an account, exchanged emails with the young woman and conducted video chats, the government said.

Rajkumar tried repeatedly to get the young woman to remove her shirt during those video chats. Eventually, she relented and exposed her chest to him, the government said.

School policy at Miss Porter's forbade any communication between students and teachers except via email through the school's information technology system.

The following November, Rajkumar engaged in a sexual relationship with the victim that lasted several months. During that relationship, the government said, Rajkumar and the victim had several video chats during which he frequently made lascivious comments to the victim and pressured her to expose herself.

An investigation of Rajkumar's computers revealed that Rajkumar had begun to harass other young girls at the school in person and through text messages. He told one female student via text message that she looked "hot" in the pants she was wearing at a school event and told another that she should create an anonymous email account so that they could do "naughty things" that were "secret," according to the U.S. Attorney's office. Rajkumar exchanged electronic messages with at least six victims, including the one victim with whom he eventually had sexual intercourse. He asked at least one of the other victims to start a physical relationship with him, the government said.

Rajkumar is scheduled to be sentenced Feb. 6 by U.S. District Judge Robert N. Chatigny. Rajkumar faces a minimum of five years of prison and a maximum of 20.

Rajkumar is now in state prison. He pleaded guilty to second-degree sexual assault as part of a plea agreement and was sentenced Jan. 7 to 10 years in prison, suspended after he serves 18 months. Upon his release from prison, he will be on probation for 10 years and must register as a sex offender for 10 years.

Farmington police and the U.S. Department of Homeland Security investigated the case. Farmington police praised Miss Porter's for its cooperation with authorities throughout the investigation.